Ultrafiltration and/or dialysis membranes are barriers which permit selective transport of solvent and some solutes across them. They are used in a variety of industrial applications, ranging from the re-concentration of dilute paint dispersions to the isolation of food products and pharmaceuticals, and in biomedical applications such as hemodialysis. Such membranes are typically produced in three general physical formats: sheet membranes, hollow fibers, and tubes. Most permselective membrane disclosed in the literature are produced by solution/coagulation processes wherein a solubilized polymer is cast as a thin film and then coagulated in a non-solvent liquid. The non-solvent is generally selected from one of many liquids which is miscible with the solvent, but which reduces its solvent power, causing loss of polymer solubility when added to the solvent. Various adjuvants may also be added, either to the polymer solution and/or the coagulation bath, for example, to aid in control of the morphology of the resulting membrane.
Polymers which have found wide applicability as starting materials for membrane manufacture are roughly divisible into three categories. First, there are hydrogel polymers, such as cellulose, chitosan, and poly(vinylalcohol). These are characterized by high equilibrium water contents, large swelling values when cycled from dry to water wet states, and uniform cross-sectional structures. Second, there are glassy engineering plastics, such as poly(amides), poly(sulfones), poly(ethersulfones), poly(carbonates), and poly(dimethyl phenyleneoxides). These polymers are characterized by low equilibrium water contents and very strong hydrophobic bonding properties. They may or may not be semi-crystalline, as in the case of polyamides. Third, there are a number of vinyl copolymers, including copolymers of acrylonitrile (AN), or charged derivatives of engineering plastics, such as sulfonated poly(sulfone) (U.S. Pat. No. 4,207,182 to Marze) which exhibit some properties characteristics of each of the first two classes described. For example, a copolymer of acrylonitrile and methallylsulfonic acid used for hemodialysis (U.S. Pat. No. 4,545,910 to Marze) shows a significant equilibrium water content, approaching the values characteristic of hydrogels, but also has strong adsorptivity for human serum albumin (HSA) characteristic of the hydrophobic bonding properties encountered when the engineering plastics are used to prepare membranes. Alkyl copolymers of AN with alkyl(meth)acrylate esters are also known for textiles and membrane applications, but these are only marginally hydrophilic, with less than 15% equilibrium water content.
The preferred choice of polymer for a particular membrane application is governed by the anticipated application. A primary consideration is the mechanism of solute separation for the particular membrane which is based upon size fractionation of dissolved solutes. The porous structure of the membranes permit the selective transfer of molecules across them if the molecules are less than, for example, 0.5 times the diameter of the pore in size, and the pores are sized accordingly. However, because of the usual similarity of sizes between the pore dimensions and the effective hydrodynamic radii of the solutes in a typical membrane, multiple collisions occur between the solutes being separated and the walls of the membrane pores. As a consequence, the nature of the membrane material can also have a profound effect on the ability of solute molecules to pass through the membrane pores. Polymers with strong adsorption properties for the solute are affected in two ways: first, adsorbed molecules will restrict the pores, thus reducing the flux of solvent through the membranes. second, adsorption of solutes affects the sieving of other solutes trying to pass through the restricted pores. (See, e.g., Robertson, B.C. and Zidney, A.L. "Protein Adsorption in Asymmetric Membranes with Highly Constricted Pores" J. Colloid and Interface Science 134: 553-575 (1990))
These phenomena are particularly noticeable when protein solutions are processed to retain larger solutes via ultrafiltration or for dialytic removal of microsolutes. In both procedures the number and effective size of the membrane pore determines the efficiency of mass transfer. A number of scientific papers (See, e.g. "Ultrafiltration Membranes and Applications", A. R. Cooper, ed. Plenum Press, 1980) have demonstrated that the adsorption of proteins on the surfaces of pores in membranes made from hydrophobic polymers is the principal source of flux reduction through a mechanism termed "fouling". This mechanism is distinct from the loss of solvent flux through solute accumulation at the membrane surface via concentration polarization effects; the latter may operate even when there is no solute adsorption leading to "fouling".